1. Field of the Invention
This invention relates to a process for preparing ammonium tungstate. More particularly, the invention concerns a process for preparing ammonium tungstate from calcium tungstate ores or concentrates that have been subjected to a reduction step.
2. Prior Art
Various tungsten compositions are known in the art. Calcium forms a tungstate mineral known as scheelite (CaWO.sub.4). Calcium also forms a tricalcium tungstate of the formula Ca.sub.3 WO.sub.6. Iron and manganese form a range of tungstate minerals varying in composition between FeWO.sub.4 and MnWO.sub.4. These minerals are generally referred to as ferberite, wolframite and huebnerite, depending on the composition of the mineral. Generally, ferberite is used to describe such a mineral containing more than 20% by weight of FeO and huebnerite, when the mineral contains more than 20% by weight of MnO. The intermediate compositions are referred to as wolframite, although this term is often used for the whole range of compositions; such use may be made hereinafter. These various minerals and tungsten compositions are important sources of tungsten trioxide and metallic tungsten.
In conventional processes for recovering tungsten values from scheelite or from wolframite ores, the ores are usually concentrated by gravity, magnetic and/or flotation techniques. The concentrates, so formed, are then processed further. For example, water soluble sodium tungstate can be obtained by fusing a concentrate with sodium carbonate or by contacting a concentrate with hot sodium hydroxide solution. Acidification of the aqueous sodium tungstate solution yields insoluble tungstic acid. Alternatively, tungstic acid can be obtained from the concentrate by leaching the concentrate with a strong acid, such as concentrated hydrochloric acid. The tungstic acid can then be dissolved in a base and processed further to recover other tungsten values.
Frequently ammonium tungstate is formed in known processes as an intermediate, which is then isolated, dried and heated to form tungsten trioxide. The ammonium tungstate is believed to be the paratungstate salt, which may be defined as having the formula x(NH.sub.4).sub.2 O.yWO.sub.3.zH.sub.2 O with, for example, x=3 and y=7 or x=5 and y=12, the value of z depending on the conditions of crystallization of the salt. This intermediate can be formed by dissolving tungstic acid in ammonium hydroxide, and subsequently can be converted to tungsten trioxide, tungsten and other useful tungsten compounds by techniques such as those described in K. C. Li and C. Y. Wang, "Tungsten," American Chemical Society Monograph 130, Reinhold Publishing Corporation, New York (1955), especially Chapter IV.
Each of the above-described processes is useful for extracting tungsten values from various tungsten compositions. However, lower levels of impurities in products produced with good yields are technically and economically desirable. For example, ammonium tungstate manufactured by known processes is susceptible to contamination by impurities. The concentration of such impurities depends at least in part on the actual process conditions employed in the manufacture of the ammonium tungstate and on the composition of the tungsten ore.